Leadless oil filled pressure transducer

ABSTRACT

An oil filled pressure transducer which exhibits reduced backpressure and utilizes a smaller volume of oil employs a glass pre-form which has a plurality of pin accommodating apertures and has an oil tube accommodating aperture. There are a plurality of contact pins inserted into the pin accommodating apertures and which extend from the top to the bottom surfaces of the pre-form. There is an oil fill tube inserted into the oil tube accommodating aperture, which oil fill tube extends from the bottom to the top surface of the pre-form with one end of the tube extending above the top surface of the pre-form. There is a glass alignment plate which has an alignment aperture for encircling the extended oil fill tube and has a sensor accommodating shaped aperture located at a predetermined position from said alignment aperture. When the glass alignment plate is being accommodated on the top surface of the pre-form by inserting the alignment aperture about the oil tube, the position of a sensor module is accurately determined based on the position of the sensor accommodating aperture. A sensor module is now placed in the shaped aperture of the alignment plate and makes contact with the pins to enable the sensor contacts to contact the pins. A header surrounds the sensor as positioned on the glass pre-form and a diaphragm is then placed to cover the top surface of the header to create a space between the top surface and the sensor. This space contains oil which is placed in the space by the oil fill tube. Due to the presence of the glass alignment plate, which has a given thickness, the volume of oil now required is much less than the volume required in the prior art, thus substantially reducing backpressure and further assuring that the sensor device is always properly aligned with respect to the pins.

FIELD OF INVENTION

The present invention relates generally to an oil filled pressuretransducer and a method for making such a transducer and moreparticularly, to an oil filled pressure transducer having a lowerbackpressure when temperature is increased.

BACKGROUND OF THE INVENTION

Oil filled pressure transducers are well known. The prior art shows suchdevices that have been used with oil, where oil or fluid is employed asa force-transmitting medium. In such devices, the pressure is applied toa transducer diaphragm, and typically these devices employ a metaldiaphragm as a force collector. Such oil filled transducers are known inthe art. For example, see U.S. Pat. No. 4,406,993 entitled, “Oil FilledPressure Transducers” issued on Sep. 27, 1983 to Anthony D. Kurtz, theinventor herein, and assigned to Kulite Semiconductor Products, Inc.,the assignee herein. See also U.S. Pat. No. 6,330,829 entitled, “OilFilled Pressure Transducer” issued Dec. 18, 2001 to A. D. Kurtz et al.,the inventor herein, and assigned to Kulite Semiconductor Products,Inc., the assignee herein.

It is well known that the pressure caused by a deflection (δ) of aclamped edge diaphragm of thickness (t) and radius (a) is given by:

$P = \frac{{16\delta \; {Em}^{2}} + 3}{3{a^{4}\left( {m^{2} - 1} \right)}}$

Where m=reciprocal of Poisson's Ration

P=pressure

E=Young Modulus

In an oil-filled pressure transducer, the sensor element is isolatedfrom the pressure media by means of a thin metal diaphragm and thepressure is transmitted to the sensor by means of a volume of oil mainlyin the form of a film between the metal diaphragm and the sensor. Thisis, of course, indicated in the above-noted patents. In any event, whenthe temperature increases, the oil expands and pushes against the metaldiaphragm thus exerting a backpressure against the sensor leading to anerror signal. This problem is known and is addressed and explained inU.S. Pat. No. 5,999,082 entitled, “Compensated Oil Filled PressureTransducer” issued on Dec. 7, 1999 to A. D. Kurtz et al., the inventorherein and assigned to the assignee herein. In that patent, the objectwas to reduce errors at very low pressure caused by the oil exerting atension on the deflecting portion of the diaphragm. A first attempt tomake a minimum oil volume transducer using a leadless sensor structurewas shown in U.S. Pat. No. 6,591,686 entitled, “Oil Filled PressureTransducer”.

The prior art describes an oil filled pressure transducer which utilizesa leadless sensor which is secured to a header comprising a glasspre-form and a header shell. The glass pre-form contains holes whichaccept header pins and another aperture or hole which accepts the oilfill tube. The diameter of the sensor was chosen to be almost as largeas the inner diameter of the shell. In this manner, there was a smallcut out over the portion of the sensor that would otherwise cover theoil fill tube. The sensor was mounted to the header using glass bonds.There was a very small space between the outer diameter of the sensorand the inner diameter header housing, which was filled with glass usedto mount the sensor. Since there were no ball bonds or gold wires in thearea between the surface of the sensor and the diaphragm the distancebetween the sensor and metal diaphragm was drastically reduced, therebysubstantially reducing the backpressure problem. However, there arestill a number of remaining problems with this construction. Making thesensor diaphragm chip the same size as the inner hole is notappropriate. It is difficult to line up the location of the header pinswith respect to the apertures in the sensing diaphragm. Using thisapproach, it is very difficult to maintain a close separation betweenthe metal diaphragm and the underlying sensor over a large diameter. Itis very difficult to orient the sensing diaphragm with respect to thepins, thus making the mounting exceedingly difficult. It is an object ofthis invention to reduce the oil volume while enabling easier sensorassembly. It is another object of the present invention to make theeffective diameter of the isolation diaphragm much larger to reduce theback pressure.

It is an object of the present invention to reduce the oil volume to aminimum volume expansion and thus reduce the error causing backpressure.

SUMMARY OF INVENTION

An oil filled pressure transducer utilizes a leadless sensor which issecured to a header comprising a glass pre-form and a header shell. Theglass pre-form contains holes which accept header pins and anotheraperture or hole which accepts the oil fill tube. There is now employedan alignment glass which becomes part of the transducer structure. Thealignment glass is self locating due to the fact that the oil filledtube is raised above the surface of the header glass. The alignmentglass has a diameter essentially equal to the inner diameter of theheader shell. It also contains a rectangular cut out which conforms tothe size of the sensor die and external from this cut out is a holethrough which the oil filled tube may pass. The sensing diaphragm ismounted to the header in the following way: The alignment glass ismounted to the header using a glass bond; the sensor is mounted to theheader using glass bonds during the same operation. In this way therectangular hole in the alignment glass defines where the header pinsare relative to the sensing element. In this way it becomescomparatively easy to mount the sensor on top of the header pins toinsure a good contact between the header pins and the sensing network onthe diaphragm.

Moreover, the diaphragm and the alignment glass can be madesignificantly larger than the dimensions of the sensor and since thealignment glass is of known uniform thickness it is easier to maintain avery narrow gap between the sensor and the metal diaphragm. This methodgives rise to a means of obtaining relatively much smaller oil volumesthan in the prior art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of a prior art leadless oil filledtransducer without a metal diaphragm cover.

FIG. 2 consists of FIG. 2A and FIG. 2B which show a top plan view and across-sectioned view of the prior art transducer 08 FIG. 1.

FIG. 3 is a perspective view of an improved leadless oil filledtransducer according to this invention.

FIG. 4 consists of a top view FIG. 4A of the prior art transducer and across sectional view FIG. 4B of the prior art transducer as shown inFIG. 1.

FIG. 5 is a perspective view of a glass alignment structure according tothis invention.

DETAILED DESCRIPTION OF THE FIGURES

Referring to FIG. 1 there is shown a prior art transducer useful inexplaining the present invention. The figure shown in FIG. 1 is FIG. 2of U.S. Pat. No. 6,591,686 entitled “Oil Filled Pressure Transducer”issued on Jul. 15, 2003 to A. D. Kurtz and assigned to KuliteSemiconductor Products, Inc., the assignee herein. That patent, asindicated above, describes an oil filled pressure transducer using aleadless sensor which is secured to a header. In any event, referring toFIG. 1, there is shown the prior art device. As seen in FIG. 1, thetransducer includes a header shell 30 which includes pins 31 and 32 andan oil fill tube 33. As will be explained the unit includes a leadlesssensor device 41 which is associated with a small oil cavity 36 ascompared to multiple oil cavities of prior art devices. It is noted thatFIG. 1 of U.S. Pat. No. 6,591,686 as indicated above, shows a prior artdevice which was prior to the device depicted in the above-noted patent.It is also indicated that the above-noted U.S. Pat. No. 6,691,686 isincorporated herein in its entirety.

FIG. 2A and FIG. 2B depict the prior art sensor as shown in U.S. Pat.No. 6,691,686 indicated above. Essentially FIGS. 2A and 2B are FIGS. 4Aand 4B of U.S. Pat. No. 6,691,681. As seen from FIG. 2A there is shown atop view of the sensor of FIG. 1 and a cross-sectional view FIG. 2B ofthe sensor. The sensor of the prior art as indicated is a leadlesssensor device as can be seen from FIGS. 2A and 2B, the header includes aheader shell 30 and a glass pre-form 32. The oil fill tube 44 isdirected to an aperture 64 in the glass 42 and communicates with anaperture 63 in the transducer. The glass pre-form 46 has a plurality ofholes which accept the header pins as hole 64 for pin 44 and hole 65 forthe oil fill tube 45. As seen in FIG. 2B the metal diaphragm 60 issecured to the header and basically covers the top portion of the sensor41. The region requiring oil is shown by reference numeral 61 and is theregion between the metal diaphragm 60 and the top portion of the sensor41. As indicated in U.S. Pat. No. 6,591,686, which is incorporatedherein in its entirety, it is shown that the distance between the sensor41 and the metal diaphragm is reduced thus exhibiting a markedimprovement in performance and assembly over prior art devices as thoseprior to that device depicted in U.S. Pat. No. 6,591,686. In any event,as indicated above, it is an object of the present invention to stillimprove the design.

Referring to FIG. 3 there is shown on oil filled pressure transduceraccording to this invention. It is noted that in FIG. 3 the samereference numerals have been utilized to depict the same parts as shownin the prior art FIGS. 1 and 2. Essentially, the transducer of FIG. 3includes a header shell 30 which includes pins 31 and an oil fill tube33. As will be explained, this unit includes a leadless sensor device41, an alignment glass 47 which is associated with a small oil cavity36, and to align the sensor 41 for ease of mounting, as compared to theprior art device. As can be seen, there is a similarity between thegeneral appearance of the prior art device of FIG. 1 and 2 and thatdevice of FIG. 3. The header 30 also has a peripheral flange 50 aboutthe top opening 51. The opening is covered by a metal diaphragm 60 (FIG.4B). The peripheral flange is of a given height which height partlydetermines the oil volume.

Referring to FIG. 4A, there is shown a top view of the sensor of FIG. 3and a cross sectional view (FIG. 4B) of the sensor of FIG. 3. The sensorof this invention is a leadless sensor device 41. Leadless sensors havebeen developed by Kulite Semiconductor Products, Inc., the assigneeherein. For examples of such sensor devices, reference is made to U.S.Pat. No. 6,272,929 entitled, “High Pressure Piezoresistor TransducerSuitable for Use in Hostile Environments” issued on Aug. 14, 2001 toAnthony D. Kurtz et al. See also U.S. Pat. No. 6,210,989 entitled,“Ultra Thin Surface Mount Wafer Sensor Structure and Methods forFabricating the Same” issued on Apr. 3, 2001 to A. D. Kurtz et al. andassigned to Kulite Semiconductor Products, Inc., the assignee herein.See also U.S. Pat. No. 5,973,590 entitled, “Ultra Thin Surface MountWafer Sensor Structure and Methods of Fabricating the Same” issued onOct. 26, 1999 to A. D. Kurtz et al.

Basically, these patents show leadless semiconductor sensor deviceswhich include a semiconductor diaphragm having a top surface coating ofan oxide layer. There are P+ sensor elements fusion bonded to the oxidelayer at a relatively central area of the diaphragm. The P+ fingerelements are fusion bonded to the oxide layer and extend from thesensors to an outer contact location of the diaphragm for each of thefingers. There is an external layer with P+material fusion bonded to theoxide layer and surrounding the sensors and fingers. A glass wafer iselectrostatically bonded at the bottom surface to the fingers and rim tohermetically seal the sensors and fingers of the diaphragm member. Theglass wafer includes a depression above the sensors and has a pluralityof apertures where each aperture is associated a separate finger at thecontact location, and each aperture is smaller than the associatedfinger lining up with the contact location. Each contact location can beaccessed via the associated aperture. The sensors contain piezoresistiveP and elements normally arranged in a Wheatstone bridge configuration.

The sensor basically is a leadless device in the sense that the headerpins of the diaphragm are coplanar with the top surface of the glasswafer and hence these transducers are extremely thin and do not have theconventional wire leads. The sensors described in the above-notedpatents do not have wires bonded to pins, but utilize the P+ fingerelements which are fusion bonded to outer contact locations as shown inthe above patents, all of which are incorporated herein by reference.

The header includes a header shell 30 and a glass pre-form 42. The oilfill tube 33 is directed through an aperture 64 in the glass 42. Theglass pre-form 42 contains five holes. Four of the holes accept theheader pins as hole 65 for pin 45, while the other hole 64 accepts theoil fill tube 33. The arrangement is like the prior art U.S. Pat. No.6,591,686 device. The pins 45 are about 0.01 inches in diameter andprotrude about 0.005 inches from the top surface of the glass pre-form42. The pin 45 is shown extending into the contact terminal present inthe leadless transducer 41. In addition, the oil filled tube 44 islocated near the outside diameter of the glass and is about 0.03 inchesin diameter. The pins 45 and the oil fill tube 44 are hermeticallysealed to the glass pre-form during firing, as well as the header shell.The entire structure as shown in FIG. 4 is subjected to heat whichcauses the glass associated with the sensor 41 to be secured to theglass pre-form 42 by means of heat and using a frit, which essentiallycauses a hermetic seal to form. This seal secures the glass pre-form 42to the glass layer 46 of the leadless sensor 41. The firing also securesthe header glass 42 as well as the glass section 46 of the sensor to theheader shell 30. The alignment glass 47 is selected to be almost aslarge as the inner diameter of the metal shell 30. The firing alsosecures the alignment glass 47 to the glass pre-form 42. There is asmall cut out 36 which is shown in the top view and also indicated inFIG. 2. The small cut out 36 is positioned over a portion of thealignment glass that would otherwise cover the oil fill tube 44.

The sensor 41 is mounted to the header 30 using a metal glass frit(conductive) to bond the pins to the sensor contacts and a ceramic glassfrit to join the rest of the sensor surface to the glass header. Thisenables the unit to then be fired using the glass frit and the ceramicglass frit to secure the parts together. Any space between the outerdiameter of the alignment glass 47 and the inner diameter of the headerhousing 30 is filled by the glass frit, which is used to mount thesensor and the alignment glass. This occurs when the unit is fired andthe glass flows and fills the voids. It can easily be seen that the onlyregion requiring oil is the region between the non-contact surface ofthe sensor, alignment glass and the metal diaphragm 60.

As seen in FIG. 4, the metal diaphragm 60 is secured to the header andbasically covers the top of the sensor 41. The region requiring oil isshown by reference numeral 61 and is the region between the metaldiaphragm 60 and the top portion of the sensor 41 and the alignmentglass 47. Since there are no gold bonds or gold wires in the areabetween the surface and the sensor of the metal diaphragm, this region61 is the only region requiring oil. Thus, the distance between thesensor 41 and the alignment glass 47 and the metal diaphragm isdrastically reduced, leaving a gap, for example, of between 0.001 inchto 0.002 inch. This small gap and the ability to reduce almost to zero,the space between the outer diameter of the sensor 41 and the alignmentglass 47 (FIG. 5) and the inner diameter of the header housing serves toreduce oil volume by at least an order of magnitude. As seen in FIG. 4Athere is an alignment glass 47. The alignment glass 47, as seen in FIG.5, basically is a glass member which has an alignment aperture 48 and achip accommodating aperture 49. When the unit is fabricated the oil filltube 33 expends above the top surface of the glass member 42. In thismanner, the alignment glass is placed so that the aperture 48 encirclesthe oil fill tube 33, as shown in FIG. 4B. In this manner, the sensoraccommodating aperture 49 is precisely positioned with respect to thepins as 45. Thus, one now places the sensor chip 41 in the pin areasurrounded by the walls of aperture 49. This assures that the sensorchip 41 is properly aligned. It further assures that the spacing betweenthe diaphragm and the top portion of the sensor is minimized because ofthe precise alignment. This reduces the gap and can reduce it to almostzero, because of the method of aligning. As one can see, the top surfaceof the alignment glass member 47 protrudes into the oil fill area thusfurther reducing the amount of oil, while the aperture 49 allows oil tobe directly coupled to the deflecting diaphragm associated with thesensor 41. Thus the use of the alignment plate substantially reduces theamount of oil required to fill the region 61. Essentially, one canreadily compute the expected deflection of the metal diaphragm for thetwo different structures and thus, the backpressure, (new FIGS. 3 and 4)and prior art (FIGS. 1 and 2).

The deflection (δ) for an increase ΔP is given by the equationδ=h×(dB/dT)×ΔT. Where (h) is the height of the oil gap, ΔT is thetemperature increase, (B) is the bulk compressibility of the oil anddB/dT is the coefficient of thermal expansion of the oil. For siliconoil dB/dT=100×10⁻⁵/K°.

Thus, for a difference of temperature of 100° K, δ=0.1 h, when h=0.001inch, δ=0.0001 inch. However, when h=0.015 inches, δ=0.0015 inches,which is 15 times larger. Since the backpressure is directlyproportional to δ, the wider gap oil filled unit exhibits a backpressure15 times larger, thus leading to a substantial error signal. It istherefore apparent that the apparatus shown, for example in FIGS. 2 and4 are extremely desirable and exhibits a marked improvement inperformance over prior art devices.

From the two oil heights assuming the following:

t=0.001 inches

E=30×10⁶ lb/in²

M=3.3

The actual backpressures with given radius are:

Wide Gap Narrow Gap .015 .001 Radius Oil Height Oil Height .10″ 2.64 PSI .18 PSI .20″ .165 PSI .011 PSI

Where P₁ is the backpressure for the wide gap case, which is the priorart device shown, for example, in FIG. 1 and in FIG. 2 and P₂ is thebackpressure for the narrow gap case, which is the improved oil filledtransducer shown in FIG. 3 and FIG. 4.

1. An oil-filled pressure transducer, comprising: a glass pre-formhaving a plurality of pin accommodating apertures and having an oil tubeaccommodating aperture, said glass pre-form having a top and bottomsurface, with said apertures extending between said surfaces, aplurality of contact pins each inserted into and associated with one ofsaid pin accommodating apertures and extending from said top to saidbottom surface of said pre-form, an oil fill tube inserted in to saidoil tube accommodating aperture and extending from said bottom to saidtop surface of said pre-form with one end of said tube extending abovesaid top surface of said pre-form, a glass alignment plate having analignment aperture for encircling said extended oil fill tube at saidtop pre-form surface and having a sensor accommodating shaped aperturelocated at a predetermined position from said alignment aperture, asensor module inserted in said shaped aperture and having contacts whichare aligned with said pins to contact said pins when said sensor ispositioned in said shaped aperture, a header having a central aperturesurrounded at a top surface by a peripheral flange with said sensor aspositioned on said pre-form, secured in said header aperture, adiaphragm covering said top surface of said header to create a spacebetween the top surface of said sensor and the bottom surface of saiddiaphragm which space is determined by the height of said flange and thethickness of said alignment glass secured to said pre-form to provide anoil accommodating area for containing a volume of oil to substantiallyreduce back pressure as compared to a conventional transducer of thesame size.
 2. The pressure transducer according to claim 1 wherein saidheight of said flange is about 0.001 inches.
 3. The pressure transduceraccording to claim 1 wherein said pins are sealed to said glass pre-formby a hermetic seal.
 4. The pressure transducer according to claim 1wherein said sensor is secured to said glass pre-form by a glass fritbond.
 5. The pressure transducer according to claim 1 wherein saidsensor and said glass pre-form is secured to said header by a glassbond.
 6. The pressure transducer according to claim 1 wherein saidalignment glass is secured to said header by a glass bond.
 7. Thepressure transducer according to claim 1 wherein said backpressure isreduced by a factor of at least 10 times as compared to a conventionaltransducer of the same size.
 8. The pressure transducer according toclaim 1 wherein said conductive pins are about 0.010 inches in diameter.9. The pressure transducer according to claim 1 wherein said height ofsaid peripheral flange is between 0.001 to 0.002 inches.
 10. Thepressure transducer according to claim 1 wherein the volume of oilcontained is an order of magnitude less than required for a conventionalsensor of the same size.
 11. The pressure transducer according to claim1 wherein said semiconductor portion of said sensor is fabricated fromsilicon.
 12. The pressure transducer according to claim 1 wherein saidsensor module is a piezoresistive sensor module.
 13. The pressuretransducer according to claim 1 wherein said diaphragm is a metaldiaphragm.
 14. The pressure transducer according to claim 12 whereinsaid sensor module is a leadless sensor.
 15. The pressure transduceraccording to claim 12 wherein said piezoresistive sensors are P+ siliconsensors.
 16. The pressure transducers according to claim 15 wherein saidP+ sensors are fusion bounded to an oxide layer on a semiconductordiaphragm.
 17. The pressure transducer according to claim 15 whereinsaid sensors are arranged in a Wheatstone bridge configuration.